Good source of info on tubes?

I have a Jolida 1501RC with GE 5751 low noise tubes. I put those in because Brendan at TubeWorld recommended them. (I didn't like the highs of the original tubes). They've been great but it's time to replace them.

I'm interested in learning more about the tubes that would be good for this amp. Specifically, I'd like to learn the differences between the 5751 tubes like the years, double/triple mica(??), gray/black(??). I don't know what any of this means.

Right now I'm looking at these at TubeWorld just because they are a decent price: Low Noise Pairs 5751 GE gray plates NOS double mica 1970's white printing original boxes
2.87/3.15 and 3.0/3.0ma.

By paying more for other tubes, am I getting a better sound? or just different?

Sorry about all the questions...

Here is a site that has some good information for you to read regarding tubes :
Brendan at Tube World and Andy at VTS can both help you. What you are looking for are low noise and closely matched tubes. Both vendors will have these in stock.

You may also want to look here for more information on 12AX7/5751 tubes:

Joe's Tube Lore
Thanks. Brendan has been great.
I'll probably go with the low noise 5751 GE gray plates NOS that he has. They're only $65.
Will I notice much difference between those and the ones that are $150?
Brent Jessie has good stuff and he is honest,good luck,Bob
Brent Jesse
Checked out audiotubes. Doesn't look like he has any GE tubes...